Oxidation/reduction (redox) chemistry of iron-protoporphyrin IX (heme) is central to many fundamental biological processes including energy transduction, redox catalysis, sensing, and signaling. Although there have been many studies of structure-function relationships in heme proteins, the relationship between heme protein dynamics and function has received little attention. In addition, our understanding of how redox- dependent biophysical properties of heme proteins impact electron transfer is not well developed. In this project, we will investigate how cytochrome c (cyt c) structural mobility modulates heme redox function. Cyts c are ubiquitous proteins displaying a range of redox-related functions. They are characterized by the covalent means of heme attachment to the polypeptide, usually to two Cys residues in a Cys-X-X-Cys-His motif. Cyts c span a larger range of potentials (from -412 to +450 mV vs. NHE) relative to cyts b (-130 to +390 mV vs. NHE), which have the same heme cofactor but lack covalent bonds to the polypeptide. To date, an explanation for this variation in range of potentials, and the ability of heme c to access extremely low potentials, has not been made. The means of attachment of heme c to the polypeptide impart on it unique structural and dynamical properties. Data on structure and redox potential described herein present a compelling case that the specific properties of heme c play an important if indirect role in redox potential tuning;we propose that the local structure and fluctuations about the c-heme motif tune iron-ligand interactions and thus redox potential. In addition, we will explore how control of the conformation of the protoporphyrin IX macrocycle itself by the c-heme attachment contributes to redox potential tuning. Cytochromes play vital roles in energy transduction in mitochondria, bacteria, and chloroplasts, as well as many archaea. Such processes are a significant source of reactive oxygen species (ROS), which are a major contributing factor to diseases associated with aging. To develop a complete understanding of events contributing to aging and cell death, and to the development and progression of a range of diseases, the fundamentals of redox chemistry in the cell must be understood. Elucidating basic mechanisms controlling biological redox chemistry is therefore of fundamental importance to biomedical sciences and human health.